This invention relates to a cubic boron nitride-based very high pressure-sintered material having very excellent toughness and wear resistance, and more particularly to a cubic boron nitride-based very high-pressure-sintered material adapted for use in cutting tools for milling hard steels such as die steels and high-speed steels, and for like machining purposes.
Most hard steels such as die steels and high-speed steels generally have increased degrees of hardness exceeding 50 in terms of Rockwell hardness (C scale) after they have been subjected to heat treatment, and are accordingly very difficult to machine successfully. Therefore, in lieu of machining, such hard steels are ground mainly by the use of grinding wheels employing cubic boron nitrides (hereinafter abbreviated as "CBN" unless otherwise specified) as main ingredients.
However, in recent years, in order to comply with increasing demands for shortening the working time, attemps have been made to supersede grinding of hard steels of the above-mentioned kinds by machining in some technical fields. To this end, it has been proposed to employ CBN-based very high pressure-sintered materials in cutting tools for machining such hard steels. However, none of the CBN-based very high pressure-sintered materials conventionally employed in such cutting tools satisfy both high toughness and high wear resistance required for machining the hard steels. For instance, those sintered materials which are satisfactory only in wear resistance can easily be chipped, whereas those sintered materials which are satisfactory only in toughness can be worn out after a short period of use. This is why those conventional CBN-based very high pressure-sintered materials find very limited uses wherein small loads are applied on the cutting edges of the cutting tools, such as finish machining by the use of a lathe. Under such existing circumstances, one would never imagine the use of CBN-based very high pressure-sintered materials in cutting tools for milling the aforementioned hard steels, because milling generally requires severe machining conditions wherein the cutting edges of the cutting tools are susceptible to far larger loads as well as far larger thermal impacts and mechanical impacts than machining by means of a lathe.